Conventional photographic materials based on silver halide are used for a large variety of applications. As is generally known silver halide materials have the advantage of high potential intrinsic sensitivity and excellent image quality. On the other hand they show the drawback of requiring several wet processing steps employing chemical ingredients which are suspect from an ecological point of view.
In the past several proposals have been made for obtaining an imaging element that can be developed using only dry development steps without the need of processing liquids as it is the case with silver halide photographic materials.
A dry imaging system known since quite a while is 3M's dry silver technology. It is a catalytic process which couples the light-capturing capability of silver halide to the image-forming capability of organic silver salts.
Another type of non-conventional materials as alternative for silver halide is based on photopolymerisation. The use of photopolymerizable compositions for the production of images by information-wise exposure thereof to actinic radiation is known since quite a while. These methods are based on the principle of introducing a differentiation in properties between the exposed and non-exposed parts of the photopolymerizable composition e.g. a difference in adhesion, conductivity, refractive index, tackiness, permeability, diffusibility of incorporated substances e.g. dyes etc. The thus produced differences may be subsequently employed in a dry treatment step to produce a visible image and/or master for printing e.g. a lithographic or electrostatic printing master.
As a further alternative for silver halide chemistry dry imaging elements are known that can be image-wise exposed using an image-wise distribution of heat. When this heat pattern is applied directly by means of a thermal head such elements are called thermographic materials. When the heat pattern is applied by the transformation of intense laser light into heat these elements are called heat mode materials or thermal imaging media. They offer the additional advantage compared to most photo mode systems that they do not need to be handled in a dark room nor that any other protection from ambient light is needed.
In a particular type of heat mode elements, e.g. as disclosed in EP 0 674 217, density is generated by image-wise chemical reduction of organic metal salts, preferably silver salts such as silver behenate, without the presence of catalytic amounts of exposed silver halide such it is the case in the dry silver system.
Another important category of heat mode recording materials is based on change of adhesion, e.g. as disclosed in U.S. Pat. No. 4,123,309, U.S. Pat. No. 4,123,578, U.S. Pat. No. 4,157,412, U.S. Pat. No. 4,547,456 and PCT publ. Nos. WO 88/04237, WO 93/03928, and WO 95/00342.
In still another particular type of thermal recording or heat mode recording materials information is recorded by creating differences in reflection and/or in transmission on the recording layer. The recording layer has high optical density and absorbs radiation beams which impinge thereon. The conversion of radiation into heat brings about a local temperature rise, causing a thermal change such as evaporation or ablation to take place in the recording layer. As a result, the irradiated parts of the recording layer are totally or partially removed, and a difference in optical density is formed between the irradiated parts and the unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and 4,291,119 and British Pat. No. 2,026,346)
The recording layer of such heat mode recording materials is usually made of metals, dyes, or polymers. Recording materials like this are described in `Electron, Ion and Laser Beam Technology", by M. L. Levene et al.; The Proceedings of the Eleventh Symposium (1969); "Electronics" (Mar. 18, 1968), P. 50; "The Bell System Technical Journal", by D. Maydan, Vol. 50 (1971), P. 1761; and "Science", by C. O. Carlson, Vol. 154 (1966), P. 1550.
Recording on such thermal recording materials is usually accomplished by converting the information to be recorded into electrical time series signals and scanning the recording material with a laser beam which is modulated in accordance with the signals. This method is advantageous in that recording images can be obtained on real time (i.e. instantaneously). Recording materials of this type are called "direct read after write" (DRAW) materials. DRAW recording materials can be used as a medium for recording an imagewise modulated laser beam to produce a human readable or machine readable record. Human readable records are e.g. micro-images that can be read on enlargement and projection. An example of a machine readable DRAW recording material is the optical disc. To date for the production of optical discs tellurium and its alloys have been used most widely to form highly reflective thin metal films wherein heating with laser beam locally reduces reflectivity by pit formation (ref. e.g. the periodical `Physik in unserer Zeit`, 15. Jahrg. 1984/Nr. 5, 129-130 the article "Optische Datenspeicher" by Jochen Fricke). Tellurium is toxic and has poor archival properties because of its sensitivity to oxygen and humidity. Other metals suited for use in DRAW heat-mode recording are given in U.S. Pat. No. 4,499,178 and U.S. Pat. No. 4,388,400. To avoid the toxicity problem other relatively low melting metals such as bismuth have been introduced in the production of a heat-mode recording layer. By exposing such a recording element very shortly by pulses of a high-power laser the writing spot ablates or melts a small amount of the bismuth layer. On melting the layer contracts on the heated spot by surface tension thus forming small cavitations or holes. As a result light can pass through these cavitations and the density is lowered to a certain Dmin value depending on the laser energy irradiated.
According to EP 0 384 041 a process is provided for the production of a heat mode recording material having "direct read after write" (DRAW) possibilities wherein a web support is provided with a heat mode recording thin metal layer, preferably a bismuth layer, characterized in that in the same vacuum environment a protective organic resin layer in web form is laminated to said supported recording layer by means of an adhesive layer.
A commercially available material manufactured according to the principles of cited EP 0 384 041 is MASTERTOOL MT8, registered trade name, marketed by Agfa-Gevaert N. V.
A drawback of the method of preparation of a thin bismuth recording layer by vacuum deposition is the fact that this is a complicated, cumbersome and expensive process. Therefore, in pending European patent application appl. No. 98201117 an alternative process for applying a thin metal layer is described comprising the following steps:
(1) preparing an aqueous medium containing ions of a metal, PA1 (2) reducing said metal ions by a reducing agent thus forming metal particles, PA1 (3) coating said aqueous medium containing said metal particles on said transparent support. PA1 (1) a transparent support optionally carrying a subbing layer, PA1 (2) a thin metal layer, PA1 (3) a protective layer or layer pack,
In a preferred embodiment the metal layer is again a bismuth layer. However such bismuth layers coated from an aqueous medium suffer in their turn from another drawback. Compared to bismuth layers prepared by vacuum deposition their sensitivity to laser light is lower. This is due to the presence of a higher degree of oxidized bismuth, and to the presence of ballast compounds in the layer such as a binder and additives improving stability, which to a certain degree hamper the formation of microspheres by the action of laser radiation.
The present invention extends the teachings on heat mode recording elements based on thin metal layers applied by coating from an aqueous medium.